1.
What is Refraction
• When rays of light traveling through air enter a
denser transparent medium, the speed of light is
reduced and the light rays proceed at a different
angle, i.e., they are refracted.
• Except when the rays are normal
Refraction in Ophthalmology
• Methods for evaluating optical and refractive
state of the eye
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2.
Emmetropia
• Parallel light rays coming from infinity are focused at the
plane of the retina when accomodation is at rest.
• Clear image of a distant object formed without any
internal adjustment of the optics of the eye.
• Absence of emmetropia = Ametropia
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3.
Progress of refractive state of eye
• Birth : +2 to +3 D
• 90% of children at age 5 yrs are Hypermetropic
• 50% of children at age 16 yrs are Hypermetropic
• After the period of growth has passed , refractive state
tends to remain stationary, until in old age a further
tendency of hypermetropia is evident.
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4.
Refractive data in adult
• Normal axial length ≈ 24 mm
• Change in axial length by 1mm = ±3D
• Refraction at corneal surface= +40 to 45(+43)D
• Change in Corneal Curvature by 1mm = ±6D
• Refraction by unaccomodated lens= +16 to
20(+17)D
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5.
Angle kappa (κ)
κ (
F
M
B
N
D
Optic axis
• M = Macula
• D= Centre of pupil, on cornea
• N = Nodal point
κ = “Between the visual axis and pupillary line, hence roughly corresponds to
angle α”.
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6.
Anomalies of the optical state of the eye
• Myopia
• Hypermetropia
• Astigmatism
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7.
What is Myopia ?
• Diopteric condition of the eye
where parallel incident rays from
optical infinity
focus anterior to light sensitive layers
of retina
when accomodation is at rest.
• K/c/o shortsightedness
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8.
Myopia – Optics
Diverging lens
Emmetropia
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9.
Optics of Myopic eye
• Far point is at a finite distance inversely proportional to
the degree of myopia
• Weakest concave lens that diverges rays just sufficiently
to focus them at the retina is to be used
• Poor visual acuity is compensated to some extent by
enlarged image size due to the nodal point being further
from the retina
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10.
Causes of Myopia
• The causes of myopia are not known.
• Epidemiological correlation suggest...
▫ Lengthy periods of close work are probably a
contributory factor
▫ There is some genetic predisposition to myopia and its
severity
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11.
• Axial
• Curvature
• Index
• Positional
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12.
Axial Myopia
• AP diameter increased to 25.5 to 32.5 mm
• 90-95% cases(commonest)
• 1mm=3D
• There may be…
▫ pseudoproptosis resulting from the abnormally large
anterior segment,
▫ a peripapillary myopic crescent from an exaggerated
scleral ring,
▫ posterior staphyloma
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13.
Curvature Myopia
• Corneal curvature steeper than average, e.g.,
keratoconus,
• Radius <7-8.5 mm (normal)
• 1 mm=6 D
• Lens curvature is increased
• moderate to severe hyperglycemia (intumescence)
lenticonus (anterior/posterior)
spasm of accomodation
spherophakia
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14.
Index Myopia
• Increased index of refraction in early to moderate
nuclear sclerotic cataracts in the elderly.
• Many people find themselves ultimately able to read
without glasses or having gained “second sight.”
• Decrease in refractive index of cortex – diabetic
myopia
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15.
Positional Myopia
• Anterior movement of the lens is often seen after
glaucoma surgery and will increase the myopic error in
the eye.
• Axial myopia of buphthalmos is countered to a large
extent due to posterior displacement of lens-iris
diaphragm and flattening of the cornea
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16.
GRADING OF MYOPIA
• Low myopia(<-3D)
• Moderate myopia(-3D to -6D)
• High myopia(>-6D)
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17.
• Congenital
• Simple or developmental
• Pathological or degenerative
• Acquired(secondary myopia)
–post-traumatic
-post-keratitic
-drug-induced
-pseudomyopia
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18.
Congenital myopia
• Since birth
• Diagnosed by 2-3 years
• Mostly unilateral
• Manifests as anisometropia
• Child may develop convergent squint in order
to preferentially see clear at its far point (10-12
cms)
• Associated with
cataract,microphthalmos,aniridia,megalocornea,
congenital separation of retina.
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19.
Simple Myopia
• Rarely present at birth, but often begins to
develop as the child grows.
• Usually detected by age 8 or 12 years in school
vision tests (school myopia)
• May increase during years of growth, stabilizing
around the mid-teens, usually at about 5 D or
less.
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20.
Pathological Myopia
• 2-3% population
• Increases by as much as 4 D/yr
• Usually stabilizes at about age 20 years and frequently
results in myopia – 10 to 20 D.
• If progress is rapid from age 15-20, likely to reach 20-30
dioptres
• Commoner in women, Jews and Japanese
• k/c/o degenerative/progressive myopia
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21.
Pathological Myopia-Etiology
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22.
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23.
Symptoms
1.Defective vision
2.Muscae volitanes
• Floating black opacities in front of
eyes
• Degenerated liquified vitreous
3.Night blindness
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24.
Signs
• Prominent eyeball-elongation of eyeball mainly affects
posterior pole and surrounding area
• Cornea –large
• Deep anterior chamber
• Large, sluggish pupil
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Elogation of eyeball posterior to equator

25.
Fundus examination
• OPTIC DISC
-large and pale
-temporal edge presents as
a characteristic myopic
crescent
-peripapillary crescent
encircling the disc may be
present
-super traction crescent
may be present on nasal
side(retina pulled over disc
margin)
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26.
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27.
Fundus examination
• DEGENERATIVE CHANGES
IN RETINA AND CHOROID
-common in progressive myopia
-chorioretinal atrophic patches at the
macula
-Foster-fuch’s spot
-Cystoid degeneration-periphery
-Lattice degeneration
-Total retinal atrophy
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28.
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29.
Lattice degeneration
Figure:
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30.
Lattice, snailtrack, retinoschisis, white without
pressure
Snailtrack Retinoschisis
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31.
Posterior staphyloma
• Ectasia of sclera at posterior pole
• Crescentric shadow 2-3 DD temporal to disc,
• Sudden kinking of retinal vessels as they dip over the edges,
• Gross atrophy
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32.
Peripheral
Degenerations
Paving stone
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33.
• DEGENERATIVE
CHANGES IN
VITREOUS
-Liquefaction
-Vitreous opacities
-Posterior vitreous
detachment(PVD)-Weiss
reflex
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34.
• VISUAL FIELDS
-contraction
-ring scotoma
• ERG –subnormal electroretinogram due to
chorioretinal atrophy
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35.
Complications
• Retinal detachment
• Complicated cataract
• Vitreous haemorrhage
• Choroidal haemorrhage
• Strabismus fixus convergence
Horseshoe Tear
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36.
1.)Optical treatment of myopia
Concave lens
-Basic rule-minimum acceptance providing
maximum vision
-Modes of prescribing concave lens
▫ Spectacles
▫ Contact lens
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37.
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38.
2.)Refractive surgery
A.Cornea based procedure B.Lens based procedure
1.Radial keratotomy(RK)
2.Laser ablation corneal
procedures
a.Photorefractive keratectomy
b.LASIK
c.ReLEx
d.ICR implantation
e.Orthokeratology
1.Refractive lens exchange
2.Phakic refractive lens(PRL)
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39.
Radial keratotomy
• Deep radial incisions in peripheral part of
cornea leaving the central 4mm optical zone
• Incisions on healing-flatten the central cornea
,thus reduces refractive power
• Correct low to moderate myopia(-2 to -6 D )
• Disadv-cornea is weakened
-uneven healing-irregular astigmatism
-glare at night
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40.
Photorefractive
keratectomy(PRK)
• Central optical zone of anterior
corneal stroma is photoablated
using EXCIMER laser(193 nm UV
flash) to cause flattening of central
cornea
• Correction for -2 to -6 D of myopia
• Disadv-pain and discomfort
-post-op recovery is slow
-residual corneal haze in
centre affects vision
-expensive
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41.
LASIK(Laser assisted in-situ
keartomileusis)
• Flap of 130-160 micron thickness of anterior
corneal tissue is raised
• Midstromal tissue is ablated directly with an
excimer laser beam
• Ultimately flattening the cornea
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42.
• Advan-
-no post-op pain
-early post-op recovery
-no risk of perforation during surgery
-no residual haze like PRK
-effective for correcting myopia of -12D
• Disadv-
-expensive
-requires surgical skills
-flap related complications
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43.
• PATIENT SELECTION CRITERIA
1.Patient >20yrs
2.Stable refraction for atleast 12 months
3.Absence of corneal pathology
• ABSOLUTE CONTRAINDICATION FOR LASIK
1.Corneal thickness <450 micrometers
2.Presence of ectasia
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44.
Refractive lenticule
extraction(ReLEx)
• All-femtolaser-vision-correction
• Lenticule of corneal stroma is extracted with
femtosecond laser
• Correct myopia upto -10D
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45.
Intracorneal ring (ICR)
implantation
• Into the peripheral cornea at approximately
2/3rd stromal depth.
• Flattening of central cornea,decreasing myopia.
• Advantage-reversible procedure
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46.
Orthokeratology
• A non-surgical reversible method of moulding
the cornea with overnight wear unique rigid gas
permeable contact lenses.
• Myopia correction upto -5D
• Used in patients below 18 years of age
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47.
Extraction of clear crystalline
lens
• Fucala’s operation
• Myopia of -16 to -18 D in unilateral cases
• Clear lens extraction with IOL implantztion of
appropriate power is the refractive surgery for
myopia of >-12D
• Complications-endophthalmitis,after
cataract,retinal detachment
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48.
Phakic refractive lens (PRL)
• Intraocular contact lens implantation for
correction of myopia >-8D
• Special type of IOL is implanted in anterior
chamber or posterior chamber to natural
crystalline lens.
• Complications-
endophthalimitis,iridocyclitis,cataract,secondary
glaucoma
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49.
3.)GENERAL MEASURES-
-Balanced diet rich in vitamins and proteins
-Early management of associated debilitating
disease
-Visual hygiene
4.)LOW VISION AIDS
-In patients with progressive myopia with
advanced degenerative changes
5.)PROPHYLAXIS
-Genetic counselling
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50.
Thank you
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